A disease is just what it says: a “dis” - “ease”; meaning something is keeping you from being at ease, or your normal state. From a system theory approach, disease is a stressor that moves the body out of homeostasis. There are many kinds of diseases that are important to human evolutions. We've already discussed genetic diseases in section 2, but here we are going to look at human variation as a response to infectious disease.

Looking at the history of humanity most of our genetic diseases were dealt with through natural selection. For most of history the most important disease we suffered from was malnutrition. People were more worried about starving to death than anything else. If a small band of hunter-gatherers got a bad infectious disease, they just all died, and took the disease with them. As population density increases we are are exposed to more diseases.

Most of the selection for disease resistance is at the molecular level. * Watch this example of evolution of immunity:

Paleopathology

Paleopathology is the study of the effects of disease on human remains from archaeological sites.

Altitude Sickness

Anything that takes us out of our biological comfort zone can cause natural selection. High altitude is associated with several stressors including less available oxygen, cold, and resource scarcity. Highlanders tend to be different than lowlanders, both genetically and culturally.

* Genes found in Tibetans that help them survive high altitude shared with Denisovians

The Epidemiological Transition

We can see human history as changes in the prevalence from one class of diseases to another.

Starvation to Infectious Disease

As hunter-gatherers the main thing that killed us was starvation. If a typical isolated band of 25-50 people caught a nasty disease, the entire band would die, and the disease would die with it.

When we started agriculture we increased population density, and became sedentary, and basically started living in our own shit, not to mention that of all our domesticated animals. Agriculture was a big trade off: we had more food and didn't starve as much, but we had to deal with more infectious diseases. Most archaeological research shows that when agriculture was introduced the population went up, but the health and life expectancy of the individual went down.

One reason bats have been an important vector in diseases like Ebola is because they can fly long distances and spread the pathogens to a wide geographic area. The other reason has to do with understanding our taxonomy, our place in the animal kingdom. As mammals, bats are so similar to us that they provide almost identical environments for the pathogens to live, evolve, and then easily cross over to humans, with only small modifications necessary. This is same reason why it's especially bad for humans to eat primates. For example, earthworms get all kinds of diseases, but their anatomy is so different from ours, that you can eat a worm and the pathogens that have evolved to live in earthworms won't be able to live inside of you.

Mosquitos don't get the same diseases as we do, they function like a shared needle, in passing blood and pathogens from one person to another.

Figure \(\PageIndex{2}\) - Female Aedes aegypti mosquito used in the experiment to test attractiveness to odors from the hands of identical and non-identical twins. doi:10.1371/journal.pone.0122716.g001

Infection to Lifestyle Diseases

We didn't evolve to sit motionless at a computer screen for hours a day. Millions of years of evolution set us up to be climbing or running around gathering and hunting for food. And for most of that time there wasn't enough food to go around. When food is too fast and too cheap we get fat. High density calories were once rare and expensive.

The Evolution of Infectious Disease: Pathogens Evolve Too

Remember that when we take antibiotics, we don't kill everything, and when the new population multiplies, natural selections means that they tend to have the variations that make them resistant to the antibiotics. You might expect this evolution to be slow because the variation from sexual reproduction (meiosis) is mostly absent, but you have to take into account the length of time of each generation. Human evolution is slow because it takes about 20 years from when you're born to when you reproduce, whereas baby E. coli bacteria are fissioning (mitosis) about 20 minutes after their born. Go back and watch the video of bacteria evolving resistance in the Mutation section.

But not all pathogens are evolving faster that our culture can keep up with. Many viruses are slow to change, and we can actually get ahead of them if we all work together.

Should you vaccinate your child?

YES!

It's amazing to think of humans actually driving some viruses into extinction. Let's pat ourselves on the back! Vaccines have an exponential effect because of a phenomenon known as "herd immunity". When the percentage of immunized people reaches a critical level, there is hardly anyone around to spread the disease to.

Pharmagenomics

Even though pathogens continue to evolve, medicine seems to be getting better because our cultural progress continues at a fast pace too. One area of growth is applying our understanding of genetics to medicine and develop individualized treatments based on a patient's genome.

Lactose Intolerance

Look back at our discussion of lactase persistence as a Mendelian trait. Lactase persistence means you keep producing lactase as an adult, lactose intolerance is the opposite, you can't digest milk In this section we are examining the same trait, except that we are trying to explain how natural selection may have caused it. What are the selective advantages and disadvantages to being able to digest milk as an adult? How does culture effect our biology? It's amazing to think of Nigeria the different rates of lactose tolerance of different people within the same country.

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